1. Field of the Invention
The present invention relates to an ophthalmic apparatus, an adaptive optical system, and an image generating apparatus using the adaptive optical system and, more particularly, to an adaptive optical system which corrects wavefront aberration occurring at an object to be detected and an image generating apparatus including the adaptive optical system.
2. Description of the Related Art
Recently, a technique of adaptive optics (adaptive optical technique) which corrects low- to high-order wavefront aberrations by an adaptive optical system using an optical element capable of actively correcting optical characteristics has been put into practice and applied to various fields. This technique corrects high-order wavefront aberration by a wavefront aberration correction device such as a deformable mirror or a spatial optical modulator by sequentially measuring the wavefront aberration of probe light or signal light with a wavefront sensor, which occurs due to variations in the characteristics of an object to be measured itself, measurement environment, or the like. The above optical element was used at first to improve resolution by correcting wavefront disturbance due to atmospheric fluctuations at the time of astronomical observation. As a field in which the effect of the introduction of this optical element is large, in addition to the field of astronomical observation, the field of eye retina examination systems has attracted attention.
As an ophthalmic apparatus, an SLO (Scanning Laser Ophtalmoscope) which acquires a two-dimensional image of the retina as a plane is known as well as a fundus camera. In addition to a fundus camera and SLO, an OCT (Optical Coherence Tomography) is known, which noninvasively acquires a tomogram of the retina. Fundus cameras, SLOs, and OCTs have already been on the market for years.
An SLO and OCT are designed to acquire two-dimensional or three-dimensional images of the retina by two-dimensionally scanning a light beam on the retina using a deflector and simultaneously measuring reflected/backscattering light. The spatial resolution (to be referred to as “transverse resolution” hereinafter) of an acquired image in the in-plane direction (transverse direction) of the retina is basically determined by the diameter of a beam spot scanned on the retina. It is possible to reduce the diameter of a beam spot condensed on the retina by increasing the diameter of a beam striking the eye. The uniformity of the curved surface shapes and refractive indices of the cornea and crystalline lens, which mainly have a function of refracting light, is imperfect. Such imperfection causes high-order aberration on the wavefront of transmitted light. Even if, therefore, a thick beam is made to strike the eye, a spot on the retina cannot be condensed to a desired diameter but rather spreads. As a result, the transverse resolution of an obtained image decreases, and the S/N of an acquired image signal also decreases in a confocal optical system. Conventionally, therefore, it has been a general practice to cause a thin beam having a diameter of about 1 mm, which is robust against the influence of the aberration of the eye optics, to strike the eye to form a spot having a diameter of about 20 μm on the retina.
On the other hand, it is reported that even when a thick beam having a diameter of about 7 mm is made to strike the eyeball by using an adaptive optical technique, it is possible to condense the beam to about 3 μm, which is close to the diffraction limit on the retina, by wavefront correction, thereby acquiring a high-resolution SLO or OCT image.
An adaptive optical system basically has an arrangement like that shown in FIG. 4. This arrangement exemplifies the confocal optical system of an SLO, which includes an adaptive optical system constituted by a collimator lens 30 and concave mirrors 31a, 32, 33, and 34 and an ocular optical system constituted by concave mirrors 35 and 36. The adaptive optical system and the ocular optical system guide a beam emitted from an illumination light source (not shown) and irradiated through an end portion of an optical fiber 9 to an eyeball 6. The anterior ocular segment such as the cornea condenses this beam on a retina 61. The reflected/backscattering light from the retina 61 propagates through the reverse optical path and is coupled to the optical fiber 9 again. The beam is then branched by a fiber coupler (not shown) and guided to a photodetector. The photodetector detects the intensity of the beam. Further scanning this beam on the retina 61 using two-dimensional scanner mirrors 51 and 52 will obtain a two-dimensional retina image.
In this case, the adaptive optical system includes a wavefront correction device 1 and a wavefront sensor 2. Both the wavefront correction device 1 and the wavefront sensor 2 are arranged in an optically conjugate relationship with the position of the anterior ocular segment (an eyeball pupil 62 to be precise) of the eyeball 6. This makes it possible to detect wavefront aberration caused by the eyeball optics in a qualitatively and quantitatively equivalent state and correct the aberration.
The reflected/backscattering light from the retina 61 has a disturbed wavefront due to the influence of the characteristics of the anterior ocular segment and propagates through the ocular optical system and the adaptive optical system. The beam is then partially reflected by a beam branching member 41 and strikes the wavefront sensor 2. The signal detected by the wavefront sensor 2 is sent to a computer 8 to generate a driving signal which cancels out the calculated wavefront aberration, thereby controlling the wavefront correction device 1. The wavefront correction device 1 corrects the disturbed wavefront in this manner to generate a wavefront with little aberration, which is suitably coupled to the optical fiber 9. The reason why the overall system has the arrangement of an eccentric reflective optical system is that a confocal optical system using a lens prevents reflected light from the lens surface from striking the wavefront sensor 2 simultaneously with return light from the retina.
According to Japanese Patent No. 4157839 which applies an adaptive optical system to an SLO, a concave mirror which forms a parallel beam to be made to strike a deformable mirror is placed adjacent to a concave mirror which receives reflected light from the deformable mirror, thereby minimizing the incident angle of light on the mirror and reducing the aberration of the optical system.
The technique disclosed in A. Roorda et al., “Adaptive optics scanning laser ophthalmoscopy”, OPTICS EXPRESS/Vol. 10, No. 9/2002 also uses the arrangement of an adaptive optical OCT using such an eccentric reflective optical system, which uses a deformable mirror as a wavefront correction device. This arrangement uses two deformable mirrors, one for the correction of low-order aberration requiring a large correction amount and the other for the correction of high-order aberration requiring a small correction amount.
The first problem in the adaptive optical system is that the size of the optical system is large. This is because this optical system needs to be designed to suppress aberration while arranging the wavefront correction device 1 and the wavefront sensor 2 so as to make them optically conjugate to each other. If the residual aberration of the optical system is large, the wavefront correction device 1 must also correct this. When correcting it as well as the aberration of the eye, the wavefront correction device 1 may lack in correction stroke which determines the correction amount of the wavefront correction device 1. In the eccentric reflective optical system shown in FIG. 4, in particular, the larger the incident angle of light on the concave mirrors 32 and 33, the larger the aberration including astigmatism. Since the diameters of the concave mirrors 32 and 33 are determined by the size of the wavefront correction device 1, it is necessary to increase the focal lengths of the concave mirrors 32 and 33 with reductions in incident angle.
Reducing the diameter of the wavefront correction device 1 can reduce the diameter of an incident beam 71 and the focal length of the concave mirror 32. In most cases, however, the above deformable mirror or a spatial optical modulator using a liquid crystal is used as the wavefront correction device 1. Many of these devices have diameters exceeding 10 mm, and only a few of them have diameters smaller than 5 mm. In addition, the correction strokes of such devices are short. The thinner a beam, the smaller the focal length of a concave mirror can be. It is therefore conceivable to use a method of making a beam with a small diameter strike the wavefront correction device 1 with a large diameter. This, however, decreases the number of segments (pixels) effective at the time of correction, resulting in a deterioration in correction performance especially for high-order aberration.
According to Japanese Patent No. 4157839, the incident angle is reduced by eliminating the space between the adjacent concave mirrors. However, since it is impossible to bring the concave mirror closer to each other, it is impossible to further reduce the incident angle. In addition, it is conceivable to use a method of using a lens system as an optical system instead of mirrors by reducing the incident angle of light on the wavefront correction device 1 to zero, that is, making the incident angle and the reflection angle be coaxial. In this case, however, it is necessary to branch incident light from reflected light by using a half mirror or the like. This reduces the efficiency to at least ¼, and hence it is impossible to secure sufficient signal light intensity in a device which examines a sample exhibiting considerably low reflectivity like the retina. It is therefore difficult to use such a method.
Under these circumstances, therefore, an adaptive optical system requires a large area of several tens square cm, and hence it is difficult to implement commercial equipment with a proper size.